Conventionally, a prismatic lithium ion battery mainly comprises a battery shell and a core having a positive plate, a negative plate and a membrane. As for a lithium ion battery being relatively small in structure and not high in capacity, the core thereof often adopts the winding structure. The process for manufacturing a core with the winding structure is relatively easy and convenient, but it is not suitable for manufacturing an over-thin lithium ion battery or a large lithium ion battery which has a high capacity and a high output power, due to the big difficulty in winding operation. On the contrary, a core with the laminated structure is widely used, because it is suitable for manufacturing various kinds of lithium ion batteries with various capacities. The so-called “laminated structure” means that the positive plate covered with the membrane and the negative plate are laminated alternately, or the positive plate and the negative plate covered with the membrane are laminated alternately, or the positive plate covered with the membrane and the negative plate covered with the membrane are laminated alternately, so as to form the core.
CN 2717037 Y discloses a lithium ion battery comprising a core with the laminated structure, wherein the battery comprises a core, a shell, a positive pole and a negative pole. The core is located in a space defined by the shell. The positive pole and the negative pole are disposed on the shell respectively. The core comprises a positive plate, a negative plate, a positive conductive terminal, a negative conductive terminal and a membrane bag 1. The positive plate and the negative plate are encapsulated within the membrane bag 1 respectively, and a loop of wide welding line 2 surrounding the plate is formed when the membrane bag 1 is encapsulated. The positive conductive terminal and the negative conductive terminal are disposed on an end of the positive plate and the negative plate respectively, and protrude outwardly from the membrane bag 1 to be connected with the positive pole and the negative pole. The membrane bags 1 having the positive plates encapsulated therein and the membrane bags 1 having the negative plates encapsulated therein are laminated alternately. As shown in FIG. 1, the wide welding line 2 is disposed along the periphery of the membrane bag 1 which is almost sealed closely. However, such a battery has the following disadvantages.
(1). The welding line may be cracked when a force is applied to the battery due to an impact such as falling down and shaking of the battery. Once the welding line is cracked, the plate may flee outwardly from the cracked opening, such that the positive plate and the negative plate may be damaged or contacted each other, which may produce a short circuit in the battery, and thereby causes a series of safety risks such as heating and explosion.
(2). Since the periphery of the membrane bag 1 is almost closely sealed, it is very difficult for the air in the membrane bag 1 to be released during the process of laminating the core. Therefore, it is difficult to laminate and press the core tightly, which may cause over thick of the core, and produce difficulty for encapsulating the core with the shell, and damage the core.
(3). Generally, the welding line 2 has a width of 2-3 mm. Because the width is big, in the process of welding, the membrane has a big area to be heated and a big shrinking appears due to the heat, such that the membrane bag 1 is easy to be rumpled and warped at the periphery thereof, and further causes the misalignment of laminating the positive plates and the negative plates, which will thereby cause safety risks to the core.